Historical Rules · 3 May 2019
Rule 805.10d
Official source text from the 3 May 2019 Rules release
As the declare blockers step begins, the defending team declares blockers. Creatures controlled by the defending players can block creatures attacking any player on the defending team or attacking a planeswalker controlled by one of those players. The defending team has one combined block, and that set of blocking creatures must be legal as a whole. See rule 509.1.